208 THE NERVOUS SYSTEM. 



distal extremity usually breaks up into branches or fibrils 

 connected with terminal structures. Axis-cylinders are of 

 epiblastic origin, developing as outgrowths from the primitive 

 nerve-cells. 



The medullary sheath, or white substance of Schwann, is 

 an envelope of semisolid material investing the axis-cylinder, 

 the presence of which is the main feature distinguishing 

 medullated from non-medullated nerve-fibres. It begins near 

 the exit of the neurite from the nerve-cell and extends nearly 

 to the distal termination of the fibre. It protects and per- 

 haps insulates the axis-cylinder. The medullary substance 

 consists of an oily, refraetile, semifluid substance called 

 my din, supposed to be supported in a reticulum or sheath of 

 a horny substance termed neurokeratin. 



At regular intervals the nerve-fibres exhibit constrictions, 

 the nodes of Ranvier, dividing the fibres into internodal seg- 

 ments. At these nodes the continuity of the medullary sheath 

 is interrupted, the neurilemma and axis-cylinder coming 

 together at these points. In each internodal segment the 

 medullary sheath exhibits a number of oblique or conical 

 markings or divisions, called the incisures of Schmidt or of 

 Lantermann; the nature of these is not definitely known. 



The neurilemma, or sheath of Schwann, is a thin homo- 

 geneous membrane forming the outer covering of the nerve- 

 fibre. It is present only in the portion of the fibres included 

 in the nerve-trunks, being absent in the cerebro-spinal centres, 

 where its place is taken by neuroglia. It also disappears 

 at the nerve-terminations. At the nodes of Ranvier it comes 

 into contact with and is cemented to the axis-cylinder, inter- 

 rupting the continuity of the medullary sheaths at those 

 points. At the middle of each internodal segment a nucleus 

 surrounded by a small amount of protoplasm is situated 

 beneath the neurilemma. This nucleus apparently belongs 

 to the neurilemma, and each nucleated segment of the neu- 

 rilemma is commonly regarded as a specialized single connec- 

 tive-tissue cell. 



Non-medullated nerve-fibres, also called amyelinic or gray 

 fibres, or fibres of Remak (Fig. 79), possess no medullary 

 sheath. The sympathetic nerves are made up chiefly of this 

 kind of fibres. They consist in general of axis-cylinders 



